They will pollinate your Pembina if the root stock is pure american or canadian plum.

If you let the suckers grow you'll be taking growth away from the Pembina.

You could also graft a scion of a plum that you know will pollinate your plum. You could graft to a sucker or a branch of the Pembina. Then you could also graft varieties that ripen at different times and or tickle your taste buds.

Here's an article on one type of graft called the whip and tongue graft. It's generally done in early spring. The varieties you would graft to your tree are called scions, pronounced without the leading s. These are generally purchased in early winter, just after the holidays and ship in either March or April. I'm looking at Burnt Ridge Nursery, their website says they have 20 varieties but only 4 are available today.

Ah, thanks for the feedback. I also should have mentioned I had taken the suckers out a month ago, planted them elsewhere, and 2 of them seemed to have taken and are growing now. Even if they aren't pure wild plum, like you said John, I could graft something to them.