How to merge two different formulas

Hi..
My name is Hiren.

I am facing some problem for adding one cell formula to another cell formula. I use COPY >> PASTE SPECIAL >> ADD and all the time formula of second cell is added with result value of first cell.
EX:
[1]
Cell D3 contains A3+B3+C3
Cell D4 contains B4+C4
Cell D5 contains A5+C5
Cell D6 contains A6+B6+C6
.
.
.

[2]
Cell H3 contains F3+G3
cell H4 contains E4+F4+G4
Cell H5 contains G5
Cell H6 contains F6+G6
.
.
.

Now I want to merge two cells formula.
Finaly I need result in H column like below :

Cell H3 should contain : F3+G3+A3+B3+C3
Cell H4 should contain : E4+F4+G4+B4+C4
Cell H5 should contain : G5+A5+C5
Cell H6 should contain : F6+G6+A6+B6+C6

Can any one please share the solution for above.
Awaiting for positive reply.
Thanks.
Hiren

When I've done this, the process looks like:
1) navigate to source cell, press F2 (or otherwise get into the edit cell mode).
2) Select the part of the formula I wish to copy and press ctrl-C to put the formula on the clipboard.
3) navigate to the destination cell and press F2 to get into edit cell mode.
4) add operator and press ctrl-v to paste the clipboard to the cell.

Thanks for feedback.
This is for one cell result. But I have to do this for more than 85000 rows.
Also all cells may not contain same formulas and I need addition of both first cell into second one.

Please suggest some formula for this addition.
Thanks
Hiren