Minimum requirement: Qubicle Basic Edition
The Extrude Tool pulls out a selection of voxels to extend the model.
Start by dragging a selection of voxels.
Upon release, transform handles will appear. You can extrude forward or backwards in the facing axis, and use the other handles to resize the area to be extruded.
Extrude the area by pulling forward or backwards and finalize the operation with Enter. The extruded area will be clipped to the edge of the Bounding Box if it extends over it.
To only extrude a selection of voxels, select the voxels you want to extrude, invert the selection with Ctrl/CMD + Shift + I and ensure Mask Mode is enabled.