No, unfortunately imbuing fireball removes the lightening, you can change the lightening level though without removing fireball.
pre imbued this weapon it started off as:
16% life leech
-27 magery
15% harm (which is now 50% hit lightening)
28% fireball
40 damage increase (whetstone this to add SC -1)
You're able to imbue any hit-spell except fireball on the weapon, it could be magic arrow, or harm. you just can't change/increase the hit-spell that was added via runic hammer. (it's no longer possible to get two hit-spells like this on a melee weapon via crafting)
These were made long before imbuing was released, using runic hammers with the 'wounding assassin spike' recipe.
There were other recipes to make double hit-spell weapons though, and most of the ones I have made were made with a special material so are unable to be imbued, (you get some message saying the item is too old to survive the imbuing process).