In principle the conversion is possible. With a properly made adapter, a Montblanc ballpoint refill can be fitted into the body of the LeGrand rollerball, since the ballpoint refill has different dimensions from the rollerball refill and needs supporting inside the barrel. Ready-made adapters are not part of the accessories range; they have to be made up individually.
In practice the solution seldom convinces. The rollerball is a capped model: every jotted note means unscrewing the cap, putting it down and finding it again, in the course of which it is easily lost, while the body, once set down, can roll off the desk, which leads to cracks in the precious resin. A twist ballpoint of the LeGrand line is a good deal handier in daily use and therefore the more sensible choice if one is going to write with a ballpoint refill anyway. The reverse course, putting a rollerball refill into a capless ballpoint body, is out of the question in any case, since such refills dry out and leak without a cap.