IClamp is specified by NMODL code, not hoc. The file is stim.mod, and you will find it in c:\nrnxx\src\nrnoc under MSWin, or, if you're using UNIX/Linux/OS X, get the tar.gz source code for NEURON from http://www.neuron.yale.edu/neuron/download
and look in nrn-x.x/src/nrnoc/stim.mod
If you are absolutely certain that you cannot accomplish what you want by assigning parameter values from hoc, then copy stim.mod into the directory where your hoc files for this particular project are located, rename the file, and also edit the file to change the name of the class (by changing
} so it won't conflict with NEURON's built-in IClamp class.