Adding additional layouts will require that you patch & recompile the kernel, I'm not aware of any Croatian (Hrvatski) patches yet.
I do believe the following files require modification, the kbd(8) program may also require some changes.. as well as reinstall the system header files.
/usr/sys/dev/pckbc/wskbdmap_mfii.c
/usr/src/sys/dev/wscons/wsksymdef.h
Example of adding new layouts, this was sent to the tech@ list in 2003.
http://marc.info/?l=openbsd-tech&m=104153165920094&w=2
You may wish to obtain a US or British keyboard, for the time being anyway.
I do believe keyboard layouts can be configured separately in Xorg, which would be another workaround solution.
Hope that helps.