Can I simple remove apache22 and install apache22-worker? I has ispmanager hosting panel with existing apache configuration (some vhosts, working as nginx backend), does all that config files will saved after remove?
AFAIK ISPManager requires Apache with itk worker. It can create start processes with specified uid:gid for each virtualhost giving you higher environment isolation as stock apache22 or event-mpm.
All your configuration would be left intact if you try deinstall current apache. Saving current configuration is prerequisite for updating packages in freebsd.
If you are concerned about apache eating up all memory after spawning multiple processes you can limit this with nginx limit_access module.