Not reliably from what I hear, bavaria. I haven't tried it because I don't like pulling loads.
There are a couple of sidelocks with trick breechplugs that were made to use pellets; the T/C Black Mountain Magnum and the T/C Firestorm. I have tried pellets in my BMMag with a musket nipple and the results weren't so great. Sometimes ignition was quick (on a par with loose Pyrodex) and sometimes there was a slight delay. I saw a problem waiting to happen so went back to loose Pyrodex, which I like better anyway. T/C must've agreed because they quickly introduced the Firestorm and, not long after, discontinued the BMMag. The Firestorm ignition is reportedly reliable with pellets, even as a flinter.
It seems a little primer charge of loose RS or P could get the ball rolling with pellets in most any sidelock, but Hodgdon cautions against mixing loose powder with pellets. I figure there must be a reason as it would increase their sales if it worked.
Bob