Does anybody if it is possible to enable and configure the DHCP server included in SME Server from the command line? So reboot is not required?
Yes, but it depends on what you want to configure.
smeserver stores many of it's parameters in flat file databases that are used from within templates to generate/regenerate the various application configuration files (in this case /etc/dhcpd.conf). If what you want to change is in one of those datbases you can change the db setting, expand the template and restart the service (or use an event to do this if it exists).
From
DB Variables Configuration in the wiki...
db configuration setprop dhcpd bootp allow
signal-event remoteaccess-update
To learn more about the templating system used in smeserver, please read the
Developers Guide (especially the SME Server Internals section)
Trevor B