I started sculpting before I finished my base mesh. I am wanting to add to the base mesh using the the mirror modifier for symmetry, but when I delete one side, then add the modifier, there is a gap in the middle in sculpt mode. How can I add to the mesh with the mirror modifier, without losing my progress on sculpting?
You'll need to align the central vertices to the object origin:
Set the 3D cursor to the origin (⇧ ShiftS > Cursor to Selected while in object mode)
Switch to edit mode (↹ Tab) and select the central verts
Set the pivot point to the 3D cursor (.)
Scale them to 0 along the mirror axis. e.g. for mirroring along the X axis, press SX0
Also, make sure your mirror modifier is first in the stack - otherwise, sometimes your object will split away from the centerline like this.