You add templates by creating Template:Name and linking to it like so: {{name}}
Templates can contain variables you may fill in yourself. The Montreal one for example uses this template:
{| class="infobox" style="width: 21em; font-size: 95%;"
|-
! style="text-align: center; background: {{Infobox musical artist 2/color selector|{{{Background|}}} }};" colspan="3" |<big>{{{Name}}}</big>
{{#if:{{{Img|}}}
|<tr style="text-align: center;"><td colspan="3">[[Image:{{{Img}}}|{{#ifeq:{{lc:{{{Landscape|}}}}}|ye s|{{min|300|{{{Img_size|}}}}}x200|{{min|220|{{{Img _size|}}}}}}}px|{{{Img_capt|}}}]] <br /><div style="font-size: 90%; line-height: 15px;">{{{Img_capt|}}}</div></td></tr>
}}
|-
! style="background: {{Infobox musical artist 2/color selector|{{{Background|}}} }};" colspan="3" | Background information
{{#if:{{{Birth_name|}}}
|<tr><td>'''Birth name'''</td><td colspan="2">{{{Birth_name}}}</td></tr>
}}<!--
-->{{#if:{{{Alias|}}}
|<tr><td>'''Also known as'''</td><td colspan="2">{{{Alias}}}</td></tr>
}}<!--
-->{{#if:{{{Born|}}}
|<tr><td>'''Born'''</td><td colspan="2">{{{Born}}}</td></tr>
}}<!--
-->{{#if:{{{Origin|}}}
|<tr><td>'''Origin'''</td><td colspan="2">{{{Origin}}}</td></tr>
}}<!--
-->{{#if:{{{Died|}}}
|<tr><td>'''Died'''</td><td colspan="2">{{{Died}}}</td></tr>
}}<!--
-->{{#if:{{{Genre|}}}
|<tr><td>'''[[Music genre|Genre(s)]]'''</td><td colspan="2">{{{Genre}}}</td></tr>
}}<!--
-->{{#if:{{{Occupation|}}}
|<tr><td>'''Occupation(s)'''</td><td colspan="2">{{{Occupation}}}</td></tr>
}}<!--
-->{{#if:{{{Instrument|}}}
|<tr><td>'''[[Musical instrument|Instrument(s)]]'''</td><td colspan="2">{{{Instrument}}}</td></tr>
}}<!--
-->{{#if:{{{Years_active|}}}
|<tr><td>'''Years active'''</td><td colspan="2">{{{Years_active}}}</td></tr>
}}<!--
-->{{#if:{{{Label|}}}
|<tr><td style="padding-right: 1em;">'''[[Record label|Label(s)]]'''</td><td colspan="2">{{{Label}}}</td></tr>
}}<!--
-->{{#if:{{{Associated_acts|}}}
|<tr><td textalign="top" style="padding-right: 1em;">'''Associated <br /> acts'''</td><td colspan="2">{{{Associated_acts}}}</td></tr>
}}<!--
-->{{#if:{{{Influences|}}}
|<tr><td textalign="top" style="padding-right: 1em;">'''Influences'''</td><td colspan="2">{{{Influences}}}</td></tr>
}}<!--
-->{{#if:{{{URL|}}}
|<tr><td>'''Website'''</td><td colspan="2">{{{URL}}}</td></tr>
}}<!--
-->{{#if:{{{Current_members|}}}
|<tr><th style="background: {{Infobox musical artist 2/color selector|{{{Background|}}} }};" colspan="3">Members</th></tr><tr><td style="text-align: center;" colspan="3">{{{Current_members}}}</td></tr>
}}<!--
-->{{#if:{{{Past_members|}}}
|<tr><th style="background: {{Infobox musical artist 2/color selector|{{{Background|}}} }};" colspan="3">Former members</th></tr><tr><td style="text-align: center;" colspan="3">{{{Past_members}}}</td></tr>
}}<!--
-->{{#if:{{{Notable_instruments|}}}
|<tr><th style="background: {{Infobox musical artist 2/color selector|{{{Background|}}} }};" colspan="3">Notable instrument(s)</th></tr><tr><td style="text-align: center;" colspan="3">{{{Notable_instruments}}}</td></tr>
}}
|}
Note the things with three curly brackets. Those are variables. When you add the template to a page, you fill them in by doing this:
:{{Infobox musical artist
:|Name = Anonymous
:|Img = noimageavailable.jpg
:Img_capt = Anonymous before being pounced by the 4chan party van.
:|Background = solo_singer (this is for categories, hence the underscore. It doesn't show up in the template itself)
:|Born = 2004?
:|Origin = /b/
:|Genre = Desu desu desu
:|Occupation = Nigra
:|Years_active = 2004-present
:|Label = R. C. Mongler
:|URL = <nowiki>[http://www.4chan.org]</nowiki>
:}}
Note that this template is ovenly complex. I never work with anything approaching this scale, thus, I cannot tell you too much about it. I also suck at CSS and Mediawiki's way of doing tables, but there are some handy tools out there that can convert HTML tables to the easier to expand Mediawiki format.
I guess you could get an external page to fetch a Wiki page, but doing so requires some knowledge of PHP. You will either have to find a coder or look around Google. There is no reason why it cannot be done, but I simply don't know how to do it.