This is possible. What you would need to do is buy a Boost SIM card and have it activated, then just get a Nextel i860 and BAM! Boost service w/ Nextel phone. No unlocking is needed.
watch how much they charge you for just the sim.. Should be around $39
If it is more buy the I215 it is $59 then you get an extra charger & battery and then can sell the i215 with out the sim to someone who damaged there phone and has a sim..