Can you enter an amount on 1040 schedule 2 line 10 manually in turbotax

Yes and no.  Go to view; forms, open 1040 Sch 2.

On line to, there will be a link when you hover over it, to another worksheet.  Click on the link.  That will take you to Part II.  Again on line 10, a magnifying glass appears, click on it.

That takes you to Form 5405; Part II line 8....there you can manually enter in an amount

More or less so.

Go to Deductions & Credits->Your Home->Homebuyer Credit Repayment and click on Start or Update.

As you go through the interview, you will be asked about how much you borrowed and how much you paid back, and TurboTax will calculate the amount. If you see a question about paying back extra, "We would like to repay more than the minimum amount due", check the box and enter a number. In my test, it put the amount I entered directly on line 10 of Schedule 2 (1040).