It doesn't make much difference if it's PES face or not.
With available tools you can convert any model to EA's .o format.
There are some restrictions however.
If we talk about FIFA07/FIFA08, the game engine has a lot of limitations.
The game divides all scene resources to static resources and dynamic resources.
Dynamic resources are those which have arguments in their names (and they can be changed during the game). For example: head models (m228__20801.o, where 20801 is argument (player id)), banners (t13__111812_0_0.fsh, where 111812 (club id), 0 (generic color 1) and 0 (generic color 2) are arguments), etc.
The game uses memory caching for loaded dynamic resources. It means that the memory for dynamic resources is allocated only once, at the beginning.
So for example, when the game (match) starts, the engine loads 25 or 26 faces (players, referees, manager). When some player is substituted and the new face must be loaded, the game won't allocate a memory for that new face, but will re-use existing memory. So it turns out, that all head models must be the same size to prevent problems with memory size mismatching.
So when working with game files, keep the simple rule: make all dynamic resources of same type have the same size.
For textures (.fsh), not only the file size must be the same, but also texture dimensions.
Creation Master authors knew about file size limitation and there's an additional file alignment performed by the tool. But it's limited to original file sizes (or similar).
If you want to insert a face which is larger than original files - you will need to align all existing faces to that size (including generic faces).
