Actually, it is possible to file a Married Filing Separately (MFS) tax return, and then later amend by filing a Married Filing Jointly (MFJ) tax return. This is true for all taxpayers, and not specific only to filers whose spouse happens to be a foreign national. While the Internal Revenue Service (IRS) allows taxpayers to file as MFS, and then later amend by filing MFJ tax returns, the reverse is not permitted as a matter of law, after the normal April filing deadline.
That said, you do have some, perhaps easier, options than to file MFS now and later amend as MFJ. First, if you already have an application in for your spouse for an ITIN or as SSN, then you could just request a simple extension of time to file your original tax return. That would give you an extra 6-months to receive your spouse's ITIN or SSN. Instructions on how to do that can be read at the following link:
https://ttlc.intuit.com/questions/3895790
Alternatively, you can just file as MFS and leave it at that. Or, a second option is to file as MFJ, and include an ITIN application along with your tax return filing. Both of these choices are discussed in much greater detail at this link:
https://ttlc.intuit.com/questions/3839499
Thank you for asking about this.
