Sure you can!
When slack boots it reads if the shell scripts (rc.whatever in your rc.d directory) are 'exec' or not.
To make them run at boot time, just:
#chmod +x /etc/rc.d/rc.whatever
Also, if you want to run them after bootup... without setting them to 'exec', you can always do this:
#sh /etc/rc.d/rc.whatever switch
Where 'whatever' is replaced with the service, and 'switch' is replaced with start, stop, or restart depending on what you want to do and which service you are using.
The command sh, is just a link to bash (by default), so you are really just telling bash to run the script directly and exit vs having another script que it on bootup.