Yes and No.
Yes, you can use the drain to change careers so to speak. No, you can't do it as you envision, but mainly in the misunderstanding in the way the levels would be drained and the racial restrictions. You don't get to pick which level gets drained, that is taken care of in the rules for Energy Drain, so you can't use the energy drain effect to basically erase your thief levels only.
The rules from the Dungeon Master's Guide for Energy Drain have the following rules for multi-class characters:
The character's experience points drop to halfway between the minimum needed for his new (post-drain) level and the minimum needed for the next level above his new level. Multi-class and dual-class characters lose their highest level first. If both levels are equal. the one requiring the greater number of experience points is lost first. (DMG, Page 73)
So as an example, your dual class character would have 20,001 XP at a minimum for a level 6 Thief. Level 5 Fighter would be 16,001. You get hit with 4 levels worth of energy drain. First to go would be your 6th level in Thief, dropping the XP for that down to 15,000, resulting in a T5/F5 character.
Now the tiebreaker rule applies, and for level 5, fighters need more XP than thieves. (16,000 vs 10,000). Drain rules state that the fighter level would be lost next, then another from the thief and finally again from the fighter. So net effect of 4 levels of energy drain on your character would leave you as a F3/T4.
Now, if your example Grunter is a human and gets leveled all the way down to 1, dual classing would be an option, but you could not get T/F/W. If Grunter is an Elf or Half Elf, then you could possibly declare as a T/F/W multiclass. That would be a DM decision, as I can't find rules declaring one way or the other when you have to declare as a multiclass. (Note: These are all based off your example character).
Note: The only other possible avenue is to get drained all the way to 0. If that happens, then a restoration or wish can restore your ability to level, and you could pick new classes. Humans still can only dual class, however, while Elves/Half Elves could multiclass.