While on mission building...

This is simple. Go into your CFS3/campaigns folder and back up the campaign you want to modify. In this case, make it non-operational by adding a _bkp tag (or whatever you use) to the .xml tag, NOT to the campaign name - or it'll appear in the menu anyway. So you have this:

fortress europa.xml
fortress europa.xml_bkp

Your working copy is of course fortress europa.xml

Open this with Windows Notepad - make this default for all .xml files if you can, as these are only camouflaged text files, in CFS3 at least.

Scroll down until you find this section:

<TechTree>
<Aircraft Type="BF_109G_6">
<Date Month="2" Year="1943"/>
</Aircraft>
<Aircraft Type="BF_109G_10">
<Date Month="4" Year="1944"/>
</Aircraft>
<Aircraft Type="DO_335A_1">
<Date Month="2" Year="1945"/>
</Aircraft>

It goes on to list all the stock aircraft.

As you can see, it's easy - copy/paste the full set of 3 lines again

<Aircraft Type="BF_109G_6">
<Date Month="2" Year="1943"/>
</Aircraft>

And change the Aircraft type to the one you want to use. Let's say the DL_Bf109g-10DC_White24.
Open the folder of that aircraft and look for the .air file. Here it is DL_109g-10_W24.air. If you remove the ".air" tag, you get the sim name/model name - paste this in over the existing aircraft name so you get this:

<Aircraft Type="DL_109g-10_W24">
<Date Month="2" Year="1943"/>
</Aircraft>

The Date line determines when this aircraft becomes available for you to fly in the campaign; spawning is controlled by the xdp file and is not affected by this line, so the dates do not need to correspond - imagine that your squadron is issued with that aircraft later than some others, for example.

Obviously you will not be getting hold of Bf109G-10 aircraft until much later, so here it should be, say:

<Aircraft Type="DL_109g-10_W24">
<Date Month="1" Year="1945"/>
</Aircraft>

Which is about right for this particular version.

And that's your lot. Save, close and off you go!