Hmm, not frames but you can do milliseconds. In a script the function would look like this:
Slip(100)
What 100 means is determine by the Slip Mode parameter under the session Functions category. Set it to Time.
With that, 100 means slip forward 100 milliseconds and -100 slips backward.
Slipping by frames would be good too, surprised that isn't there.
Depending on your needs, there is also StartPoint which sets "frame 0" of the loop to wherever it is playing at the moment. You won't hear anything right away, it just moves the start point, but when you then use Start or any other function that retriggers the loop from the beginning it will use the new start point.