is it possible to add addon models to player models through xsi mod tool 4?

If you mean you want to edit player models to add additional geometry then you can do this in any version of XSI be it the free Mod Tool or commercial. If you choose to do this in the free Mod Tool then you are simply adding an extra step to the exporting process.
You'll need to get your scene out of Mod Tool and into a version of XSI that is compatible with the PandemicBF2.xsiaddon to export it to msh since the data to be exported will consist of enveloping and weighting which 3rd-party tools don't support.

If you are talking about addons like visors and backpacks all you need is:

XSI 6.0 or 7.5 Mod Tool Free Download ( not Foundation trial version )
http://www.gametoast.com/forums/viewtop ... 36&t=14467

and

MeshEx_beta_3
http://www.gametoast.com/forums/viewtop ... 64&t=15082

I use these two for getting all my player addons, weapons, and props ingame and it works rather well.

But, if you want to do full new player models you will need to do as Ace said then.

I'll see what i can do with it. thank you all.