Theoretically it could be possible for NPCs to throw grenades but it's locked for some reason and there's no way to enable it without changing the executable. Although I discovered that there's one exception - weapon homie actually throws grenades. He has been removed but the code is still present in TableTemplates section of 'homies.xtbl' file, you can replace any other homie to check that (just make sure not to replace entry name).
Code:
<Homie>
<Name>weapon_homie</Name>
<Display_Name>HOMIES_WEAPON_HOMIE</Display_Name>
<Humans>
<Human>
<Character>npc_saint_male_soldier</Character>
</Human>
</Humans>
<Vehicles>
<Vehicle_Entry>
<Vehicle>van_2dr_riot02</Vehicle>
<Weight>100</Weight>
</Vehicle_Entry>
</Vehicles>
<_Editor>
<Category>Entries</Category>
</_Editor>
<Flags>
<Flag>give random weapons</Flag>
<Flag>requires saints</Flag>
</Flags>
<Image_name>ui_homie_zimos</Image_name>
<Blocked_for_Mission></Blocked_for_Mission>
<Display_Desc>HOMIES_WEAPON_DESC</Display_Desc>
<Audio>PHONE_WEAPON</Audio>
</Homie>
I'm not sure what's required to make a regular homie throw grenades but it's probably 'give random weapons' flag. I'm also not sure if it has any other effects.