There are a few places where you can enter your DMR ID in Pi-Star.  My guess is that you're trying to enter the extra digits under the "General Configuration" area.

Instead,  try entering your base DMR ID in that field and then using the drop down to select a unique suffix under the DMR ESSID in "DMR Configuration.

If using a single DMR Master, you'll only have one place to do that.  If using DMRGateway, then you could have several places to select different suffixes for individual networks.