Do FAA knowledge test endorsements have an implicit expiration date? If not, can I add one explicitly?
The last that I looked into it, there were no expiration dates on written test endorsements by CFI and GI.  However, the FAA has made it clear many times that instructors are free to add conditions to endorsements.

For example, when I endorse a student for solo, I will put limitations like:
1.Prior phone call to instructor, 2. surface winds less than 14K, and within 45 deg of runway heading, 3.Five miles vis with VFR conditions.

Of course solo endorsements have implied expiration dates. But you could endorse J Pilot for a instrument written, and add "Endorsement expires Jan 1, 2018."

Since these things are vetted regionally, if you have any concerns, you can always call the FSDO and talk with an Ops Inspector there.

I did that once when I signed a guy off for a Flight Review, and wanted a condition on the sign-off. The pilot agreed with the condition, and I ran it past the Safety Program Manager who was an Ops Inspector, and his words were, "Absolutely! Any restriction you think is appropriate is fine to add to the endorsement."  That was sometime ago, but I have no indication that the policy/latitude has changed.