I am just a newbie myself, and I don’t use (or even own) a pump.
However, I’m going to guess that it’s probably safe to pump, but you might not see the results. With all of the information on this great site that I have read, it seems like so many people who have been doing PE for a while stress the importance of conditioning your penis, not only to handle the stress PE can put on it, but to also maximize gains.
Therefore, I’m going to assume that more advanced (and strenuous) activities, such as pumping, would be far more effective once the penile tissues are conditioned to being put under increased levels of stress.
Again, I’m a newbie to this, but with what I know about the body (in conjunction with what I’ve learned on here) this seems to be a logical response to your question.
Any other experienced guys out there have an opinion on this?