Switching prepaid Virgin Mobile number to prepaid Sidekick T-Mobile?

Yes,you will do it through the network porting.
For porting,try to do the following steps.
1.You will need your PAC(Porting authorisation code) from your current network service.
2.Then contact your new mobile service and tell them your PAC.They will provide your new sim card.
After getting your new sim card,Make sure your Sidekick phone is unlocked one.If it's restricted by single network means,try to unlock your mobile before swap your sim.For unlocking visit WickedUnlock.com