We call these two-way traders and yes, you can.
Find the short code for the existing trader - if they exist as a supplier and you want to create them as a customer also, then go to customers and click on NEW – type this short code in, if you have the right one then their details will automatically be populated once you click on Tab.
Now check the box “Is Customer” and save it.
BEWARE – if you have lots of Sales AND Purchase invoices for this one trader and you NEVER contra between the two, it may be better to set two traders up with slightly different names (LANDC and LANDS). However if you wish to contra (Pay a net amount off) then we recommend a single trader that is a two-way trader.
It is NOT advisable to change a two-way trader to become a supplier or customer only, and used this way.