Sort of.
An external source can only include functions (although, you can include
global variable to make variables global).
In DBC, you should really only #include at the beginning and not in a loop, as the compiler puts the functions where you call #include from. Calling it from within a loop will waste loads of time (and may throw up an error).
The page file extension is .dba although I dont know if its particularly picky about the extension (ie: call it whatever you want, just make sure its a valid name)
-----
Main source
#include "extension.dba"
sync on : sync rate 0 : backdrop on
print ExtensionCommand()
wait key
#include source
function ExtensionCommand()
thestring$="External command"
return thestring$