Adding more information to a pre-exsting cell formular - excel

Question

I belive that this is possible with my limited knowldge of vba. I am trying to add more information to a pre-exsiting formula within a workbook.
Currently the formula is as follows, in Cells(SnapshotStartRow-2,11) the formula is =SUM('GK034'!B5 + 'GK034'!B21 + 'GK034'!B34)
The next stage is add an extra cell addition to the equation to look like
=SUM('GK034'!B5 + 'GK034'!B21 + 'GK034'!B34+ 'GK034'!B50)
My idea was to use this code below;
With Worksheets("Snapshot")
sAddress1 = .Cells(SnapshotStartRow - 2, 11).Address(0, 0)
End With
With Worksheets(SnapshotArray(i, 1))
sAddress2 = Worksheets(SnapshotArray(i, 1)).Cells(NewEngineRowNumber + 2, 2).Address(External = True)
End With
With Worksheets("Snapshot")
sFormula = "=Sum(" & sAddress1 & "+" & sAddress2 & ")"
.Cells(SnapshotStartRow - 2, 11).Formula = sFormula
End With
Some info on this code,
SnapshotArray(i,1) is there a reference for finding the sheet and contain the value GK034
SnapshotStartRow is used to defined a previously needed row reference and is equal to 125.
NewEngineRowNumber is used to define a previously needed adrress and is equal to 48
The idea behind the code was to take the current formular and mearly add the new cell to it.
Howevver there were a few issue that I failed to forsee,
Firstly, the 1st with loop mearly took the value of the cell rather than the exsisting formular.
Secondly the equation failed to remember which sheets the data was taken from.
Once i ran the code I ended up with, in the correct cell (The only part of the code that worked)
= SUM(K123 + $B$50)
Where K123 is the correct cell that use to contain the formular and B50 is desired cell on a different worksheet.
However K123 is now 0 and $B$50 dosen't refer to the desired workbook.
If anyone is able to help me that would be greatly appriciated. I will keep updatingthe post as i try new methods.
having done some more resurch I belive I need to extract the current formula as a string, which I belive can be done with a function, below.
Function CellFormula(Rng As Range) As String
CellFormula = Rng.Formula
End Function
Then could you add to the string the new code `$b$50 with the correct worksheet, then somehow paste the new string into the formula bar? Is that even possible?
From QHar comments, I tried to concatenate the worksheet name using,
With Worksheets(SnapshotArray(i, 1))
sAddress2 = Worksheets(SnapshotArray(i, 1)).Name & .Cells(NewEngineRowNumber + 2, 2).Address(External = yes)
End With
This didn't work either and noticed that parts of the equation were missing, I tried to add them manually and this failed to work, see below.
With Worksheets(SnapshotArray(i, 1))
sAddress2 = "'" & Worksheets(SnapshotArray(i, 1)).Name & "'!" & .Cells(NewEngineRowNumber + 2, 2).Address(External = yes)
End With