Exif data for lens?
EXIFtool will allow you to append this information into the EXIF data if you so desire.
I did it for a while, but I find it far easier to simply add keywords for lens data for my adapted manual lenses. You could do the same for aperture setting.
It's not as convenient in use as adding to the EXIF data as various tools (Lightroom, Bridge, etc) won't be able to filter on it as easily, just a lot easier to add in. I find I only rarely need it so the simple convenience of adding it as a keyword works fine for me.
|Douaumont Ossuary by Eric 54-BNF|
from Armistice Day
|Silhouette at sunset by Jill Hancock|
from Portrait Lens (around 80mm or equivalent - please check the full rules)