Can the POS ask the customer for a tip?

Yes. Under Settings → The POS, Tips, tick Ask for a tip when taking payment and set up to three percentages and up to three fixed amounts. The Charge screen then shows No tip, your choices and Other, and the tip goes on top of the bill: the card machine is charged the bill plus the tip, cash covers both. A cash tip stays in the drawer and is counted at closing, a card tip is in the card machine's batch, the receipt prints Tip and Total paid, and every shift and day report shows the tips by cash and card. The POS and the counter tab in the docs covers tips.