For the front you can change the fork. This is assuming that you have a threadless headset. I don't think there is anyone making threaded shocks anymore.

For the rear I have seen an ad for a rear suspension adapter. I don't know who makes it. It bolts onto the seat stays and chainstays and mount your rear wheel back a little further on the suspension part. It looks like a crazy thing to me.

You could also go with a suspension seatpost. This has more to do with making the ride more comfortable. It will not keep your back tire on the ground more.