Hi mathsanches,
As parnassus said, you would lose any configuration of the switch that you would add to the VSF stack, however whatever you have on the commander, or the existing switch, will retain its configuration, only the port numbering will change with the added stack member ID (I.E. 1/23 instead of 23). You can use the strict provisioning method to force one switch to become the commander and the other to become standby. In that case, the commander would retain configuration and the standby would lose it's configuration, joining the commander as one logical switch in the stack.
Please refer to the Management and Configuration Guide which shows how to setup a VSF stack, starting on page 728.
https://support.hpe.com/hpsc/doc/public/display?docId=a00050273en_us