If you need a lower Final Gravity because you're missing your target final gravity AND you were all-grain brewing, it sounds like you might have mashed at too high of a temperature, resulting in a higher level of unfermentables. If this is the case, adding more yeast won't help.
If that isn't the case (you didn't all-grain brew at high mash temperatures), but your recipe is still calling for a lower FG - I think we might need some more details . . .
What was your target Final Gravity?
What style of beer is it?
What was your mash temperature, if any?
What was your fermentation temperature?
How long did the beer ferment to get to 1.020?
Are you trying to get a higher ABV?
The high OG makes me think you're brewing a super-hoppy IPA or really huge malt-monster. 1.020 is a perfectly fine number for a 1.080 OG.
All of the above being said, I still consider myself "new" to brewing, so take the advice of more experienced homebrewers, if they post.