I´ve been wondering....?

Heck...I´ve been killin my brain about this....

IF I pick up Jaheira and Minsc so I can use them later on...They´ll gain Exp. from my lvl or so....but if I boot them and pick them up later in Athakla will they then be of higher lvl.???

I mean does the computer generate the lvl. for each time an NPC join or is it only the first time???

Only the first time, and there is a limit. If you are level 20 when you recruit someone, he might start at level 8-10 max.

Oki....so theres no way to pick up Jaheira at a higher lvl.
I want my sorcerer to gain as much exp through the dungeon as possible

There are some creatures that are leveled, and some that are fixed. Leveled are those that have several creature files.

When you enter an area for the first time, the game fills it with leveled creatures, depending on your current level. It chooses the creature file that corresponds to your level.

NPCs' levels are usually between 7th and 12th. NPCs in ToB are 12th level, but they have 2,500,000 XP, so they immediately level up.