If I file my taxes now, can I add another W2 at a later time or next year if I haven't received it yet?

NO. 

Every 2019 W-2 that you received must be reported on your 2019 tax return, even if they are for small amounts.  Remember that each one of those W-2’s has your Social Security number on it, and that income was reported to the IRS by the employer.  You do not want to get in trouble with the IRS for under-reporting your income.

ALL of your W-2’s must be entered on the SAME tax return.  After you enter the first one, you click Add Another W-2.

W-2's come from your employer, and they have until Jan. 31 to issue it.  Some employers allow you to import the W-2 through the software, but often you still need information from the actual W-2 just to import it.  So waiting for your employer might be your only choice now.

https://ttlc.intuit.com/questions/1934390-how-do-i-import-my-W-2

https://ttlc.intuit.com/questions/2588751-why-can-t-i-import-my-w-2

https://ttlc.intuit.com/questions/2938482-how-do-i-take-a-picture-of-my-w-2-in-turbotax-online

All W-2's should be entered on the tax return before it is filed.

Unless you want to have the IRS contact you for not reporting all of your income and go through the procedures of correcting the filed tax return.  Instead of waiting for any tax refund which should only take a few weeks after filing a complete tax return, you will have to wait for months to get the return sometime in May or June or even longer

If the tax return is accepted then you will have to amend the tax return to make your changes. An amended tax, Form 1040X, return can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.