Yeah, change the cell type. You should be able to do it either directly from the mapper or from code using TethysGame::SetCellType.
The cell types are:
0 (0x0) Fast Passible 1 - rock vegetation
1 (0x1) Impassible 2 - meteor craters, cracks/crevasses
2 (0x2) Slow Passible 1 - lava rock (dark)
3 (0x3) Slow Passible 2 - rippled dirt/lava rock bumps
4 (0x4) Medium Passible 1 - dirt
5 (0x5) Medium Passible 2 - lava rock
6 (0x6) Impassible 1 - dirt/rock/lava rock mound/ice cap/volcano
7 (0x7) Fast Passable 2 - rock
8 (0x8) North Cliffs -
9 (0x9) Cliffs - High side -
10 (0xA) Cliffs - Low Side - cliffs, low side (and middle too)
11 (0xB) Vents and Fumaroles - fumaroles (only passable by GeoCons/RoboMiners?)
12 (0xC) zPad 12
13 (0xD) zPad 13
14 (0xE) zPad 14
15 (0xF) zPad 15
16 (0x10) zPad 16
17 (0x11) zPad 17
18 (0x12) zPad 18
19 (0x13) zPad 19
20 (0x14) zPad 20
21 (0x15) Dozed Area - bulldozed, non tubed area (buildings are tubed underneath) - does not include buildings
22 (0x16) Rubble
23 (0x17) Normal Wall
24 (0x18) Microbe Wall
25 (0x19) Lava Wall
26 (0x1A) Tube0 - tubed (buildings are tubed underneath, including docks) - includes buildings
27 (0x1B) Tube1
28 (0x1C) Tube2
29 (0x1D) Tube3
30 (0x1E) Tube4
31 (0x1F) Tube5
The strings were taken from somewhere in Outpost2.exe, so don't go asking me what exactly they mean. I don't know unless it's obvious.