How to create conf file for include it in extensions.conf?

In extensions.conf if i want to add lots of members/stations then it become slower process.
I got info that we can use following line in extensions.conf.
#include filename.conf

for that what should i write in that ‘filename.conf’?
I am trying to keep sequencing like
0 to 1000 stations entry in temp1.conf
1001 to 2000 stations entry in temp2.conf
like this…
Should extensions.conf default lines repeatedly write in those files or only
extern==> ???
lines in those files?