I am not realy sure how to make that devices connected to wifi will seamlessly change AP.
Capsman (currently) doesn't do anything about roaming. What capsman does is to provision CAPs wireless interfaces. Which is handy because it's easy to provision all of them using identical security settings (SSID, PSK, security protocols, etc.). If you do that manually on all APs, then end result will be exactly the same. Just make sure that wireless interfaces with same configuration are all members of same IP subnet (i.e. served by same DHCP server and using same IP gateway).
Historically capsman UI was a bit different than UI of wireless interface. Additionally there were things that weren't configurable via capsman. So many people avoided using capsman unless the number of APs was larger (and thus hard to maintain manually). Things are about to change with newer APs which run wifiwave2 driver. UI of this driver is very similar to that of capsman (with similar limitations) and "weirdness" of UI won't be excuse not to use capsman anymore.
However, until capsman2 (needed to provision wifiwave2 APs) becomes stable (it was only introduced in v7.7 a few days ago), one can still configure a few APs manually and enjoy roaming (in the imperfect way we know it now) just the same.