Why do you want to do this?
Process variables are java Objects and Objects represent the "variable type".
JDPL is not exactly XPDL compatible.

@Nib:

1: e.g. to do some checking initially. If the variable has never been assigned any value (java object) it is typeless
2: do do some form generation (it will be stored in the gpd.xml now, but I (personally) are in favour of putting it in the jpdl file, just like the readonly, write, required
3: ...

@sumit: you can't, not in a simple way (one solution is to write an actionhandler to define and create them in one way or another whe the process starts)