The pass line is a contract bet, so once you put $ down - it has to stay. You can add to it. This is usually done when you want more odds. The reverse is true of the don't pass line, you can pull money off at any time - however, you cannot put the money back once the original has been pulled off.

The reasoning is this; On the come-out roll, money on the pass line is a favorite over money on the don't pass line. Once a point is established, money on the pass line is the underdog vs. money on the don't pass line. The 7 will come out more frequently than any other number. So they want money on the pass line at all times.

As far as walking up to a table in the middle of the roll, you can place the point using the pass line. The money is offset, using the line nearest you. However, you could just put money on the pass line - but that makes no sense. Suppose you walk up to a $10 table, and the point is 9. You put $20 on the pass line. If the 9 is hit, you win even money. However, if you place it using the pass line - you will receive $28 or $29 (depending on the casino).

As a side-note, when I was in Tunica for the WPO - I saw for the first time, a crapless crap game. 2, 3, 11 and 12 COULD be your point. It was quite disgusting if you ask me, I saw noone hit a 2, 3, 11, or 12 once they established it!