You didn't specify what trigger you intend to use so I'm not sure what your intended usage is. An example below can be used on the after registration and after profile update triggers to copy over when registering or saving profile.
Code:
UPDATE `#__acymailing_subscriber`
SET `cell` = '[cb_cell]'
WHERE `userid` = '[user_id]'
Please note the above is just an example. Be sure to test thoroughly. To do an initial move from CB to AcyMailing on mass scale I suggest running a query directly through phpmyadmin. The below for example should do this ok.
Code:
UPDATE `jos_acymailing_subscriber` AS a
, `jos_comprofilerr` AS b
SET a.`cell` = b.`cb_cell`
WHERE a.`userid` = b.`user_id`
Please note the above is also just an example. If your table prefix isn't jos_ then be sure to replace it with the correct prefix.