The archive package has a pre and post command to execute - look on the misc tab. The probe configurations are just text files - modify the text file and restart the probe is essentially the model to follow. If you want to stay within nimsoft, the pu command allows you to send commands and retrieve data from nimsoft. Look at the controller callbacks (in IM, you can highlight the controller and press Ctrl-P to get a GUI based interface to PU or you can run pu with no command and it will list them for you). There you will find a callback probe_config_set that will set your log file value. if you look at the logmon probe, there is a _restart command, that will restart logmon and reread the cfg file.
Or you can look to see if you can reach the log file by pattern instead of absolute - logmon can work with ?:\logs for instance. Or if you have something that has some rigidity to it, you might be able to do something like "..\..\..\mylogs" so that you essentially find the log relative to the install location of Nimsoft.
-Garin