Do they smoke cigarettes indoor or outdoor? If outdoor, I can't see why it'd be a problem. Ask anyway, just to be polite and respect them. If indoor then there could be a problem--they may not want to smell something foreign (even if many of us reading this would rather smell a cigar or pipe over cigarettes).

The biggest potential "problem" in my mind is the difference in smoking duration, if they go out to smoke or otherwise won't be smoking where the main socializing is. What I did once at a party where a few friends went outside to smoke a cigarette every once in a while was bring my pipe and load about a quarter of the bowl when they went out so it lasted about the same amount of time. I think I ended up smoking about a bowl broken up into three or four sessions.

If they're smoking indoor and are cool with you also smoking a pipe or cigar indoor with them, then it should be fine smoking for however long you want.

ETA: From my limited experience, people have tolerated or even enjoyed pipe smoke (even non-aromatics) much more than cigar smoke. So, they may be more open to indoor pipe smoking.