Upgrading Greens, your opinions please....

I have;

Zelnehlun's Longbow (green)

Piercing damage 15-28 (reqs. 9 marks)
Dmg. +15%
Armor -10 while attacking
Armor pentration 20% (Chance 20%)
Health +30

Would you add either an "of fortitude" or "of enchanting" grip to it, or a string upgrade?

Also have (for runner build);

Kepkhet's Refuge

Energy +10
Fire dmg 11-22 (reqs. 9 DF)
HCT Prot. Prey. spells 20%
HSC all spells 20%
Energy +5
Enchants last 20% longer

Would you add a "Hale staff head" and/or another "of enchant" ? Do they stack?

Does adding to the greens destroy their original stats or enhance?

What do you mean by adding to greens? They've already got Hale/Fortitude/Enchant etc.

Have I been sleeping, and we can mod them now?

You cannot upgrade green weapons.

Did not know you could not upgrade. Thanks.

Looking at your stats:

Your bow is a sundering bow of fortitude
Your staff is an insightful staff of enchanting

As said, greens can not be changed as they each come with inherent mods and upgrades.