For the N900P, the method I have used is rooting and replacing apns-conf.xml in etc folder after root. The new apns file is of 245 kb as against the sprint file of 6-8kb. This starts GSM data without any manual addition of APNs. The second option is rooting and editing the build.prop.
ril.sales_code=LOL
ro.csc.sales_code=LOL <- this will bring up the APN menu for manual input.
After switching to GSM from CDMA, and going back to CDMA gives 'insert SIM to get mobile networks'. Dial ##3282#, SPCODE, 1x-hdr selection can do it. For this use this application from playstore https://play.google.com/store/apps/details?id=diewland.settings.mobilenetworks&hl=en
More easier methods may come up.
apns-conf.xml
diewland.settings.mobilenetworks-1.apk