I want to use a disk that i use with windows before

To add it to your existing array, the disk needs to be match the size of one that's already in the NAS (or be larger than the biggest disk).  If the size isn't suitable, you'll need to switch to FlexRaid and create a new jbod volume.  It won't be protected by RAID redundancy, so it would be best to use it for data that is somewhat less critical - for instance, backups instead of primary storage.

To be clear, you will lose the data that's on the disk now (but not the data that's on the NAS).

Assuming a suitable size, you hot-insert the disk (add it to the NAS with the system running).  After a brief disk test completes, you select the disk from the graphic on the volume tab, and then select "format" on the right.  If you are using XRAID, the system will automatically add it to the array after the formatting completes. 

If you are using FlexRAID, you will need to add it manually (for instance, selecting the disk and creating a new jbod volume on it).