I don't think you can with regular perks since they come from leveling up. You might still get the special perks or quest perks since these come from special actions or interactions with a character and completing certain quests.
If your so inclined here's a list of perks. http://fallout.wikia.com/wiki/Fallout_3_perks
You max levels at level 30. You can keep on playing forever, however you will not gain experience anymore, and will not level up again. In order to get the other perks you must start a new game.
If you are playing on a console, you are maxed out at level 30. However, if you are playing on a PC, you can mod up to level 100. I an currently playing with a mod to max my level at 50.
If you are playing on a console, you are maxed out at level 30. However, if you are playing on a PC, you can mod up to level 100. I an currently playing with a mod to max my level at 50.
I suggest you check out fallout3nexus.com. It will open a whole new gaming experience for you. (sorry, you will have to enter the above manually, I don't know how to post links.)
If all you want are the perks then just add them via the console - it'd be simpler than adding new levels. You can use the player.addperk command and refer to that earlier link for the IDs to use. Alternatively, just manually advance yourself a level using the advlevel (or was it addlevel) command, and you can then select the perk you want.
I suggest you check out fallout3nexus.com. It will open a whole new gaming experience for you. (sorry, you will have to enter the above manually, I don't know how to post links.)