In my opinion, the best is to design the form from the Infopath Integration Wizard in K2 Blackpearl Studio (in the wizard, there's a step where you can edit the form). Otherwise, if you modify the form outside K2 Studio, you will have to re-bind it to the process (still using the Infopath Integration Wizard) as K2 only stores a copy and not the form template itself. But this last method can be useful if you want to keep a "clean" copy of your form template somewhere (without all the datafields added by K2).
Are there some problems when i modiy the form outside k2 studio. I don't know exactly how it is possible to rebind it to the process. I havn't done this before. Is there something I have to consider?
No, rebinding the form to the K2 process just consists in running the Infopath Integration Wizard again, like if you wanted to change the associated form. I don't know if you worked with K2.net 2003 but it was almost the same: modifying the form outside the K2 environment was discouraged but you had the possibility to rebind a form template if necessary. Personaly, I never encountered any problem.