Wilbur has particularly bad import options, there are tons of them, just none happen to be ASCII Grid (ASC).

Terrain builder → Wilbur:

1. terrain builder export heightmap to ASC
2. L3DT import ASC
3. L3DT export HFZ (L3DT)
4. Wilbur load HFZ

Wilbur → terrain builder:

1. Wilbur export lat/lon txt
2. rename txt to XYZ
3. Terrain builder import XYZ

Or the longer route without renaming, export HFZ, import to L3DT, L3DT export ASC, terrain builder import ASC.