Can I force the phone to roam? For example. If I have 1 bar and my friend with Verizon has 5, can I force it to roam on Verizon's network so I get better signal?
No extra charge if your plan includes roaming. I happen to have an older plan from 2003 that does not include roaming.
Just go to the phone's menu and look for settings > roaming > set mode > roaming only (or something close to that, it varies by phone)