Does you cruiser have a breacher barrel?
The 500 breacher models use 590/835 mag tubes and the heavy-walled barrels are actually the same as on the 590a1. If that's the case, then Nordic Components, S&J Hardware, Choate, and a few others make a +2 extension that'll fit.
If not, then unfortunately no.
The standard model 500 barrel/tube set-up does not accept extensions. To run a full sized 7 round mag tube you would have to switch to a 20" barrel.
Your only other option is replacing your current 500 tube with a model 590/835 mag tube & cap and switching to an 18.5" 590a1 barrel...