In a quick search, I found an article talking about a Lightroom plugin named LensTagger.
The interesting part is at the bottom of the description page there are screenshots where you can see the Exiftool command for your purpose.
-Lens="x" -LensModel="x" -LensType="x" -FocalLength="x" -MaxApertureValue="x" -FNumber ="x" -ExposureTime="x" -FocalLengthIn35mmFormat="x"
Where x is the value you would like to write for that field.
PS: Make a test on a copy of your file, not on the original file.
You can try:
exiftool -Lens="MyOwnLens" -FocalLength="75mm" image.arw
To check a file just type
exiftool -g image.arw
-g option will separate informations into groups for a better reading.
If you need more help just ask.