Easy - open up the campaign xml with notepad, and scroll down till you find this:
<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>
<Aircraft Type="DO_335B_2">
<Date Month="3" Year="1945"/>
</Aircraft>
<Aircraft Type="FW_190A_5">
<Date Month="2" Year="1943"/>
</Aircraft>
<Aircraft Type="FW_190A_8">
<Date Month="11" Year="1943"/>
Simply add in the aircraft you want, taking care to respect the architecture of the file...
<TechTree>
<Aircraft Type="My_Spitfire_MkIXc">
<Date Month="6" Year="1944"/>
</Aircraft>
<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>
Save and you're off. (Colour for illustration purposes only!)
The aircraft name to use is the xdp file name minus the .xdp tag, NOT the folder name.