Is it Possible to have have one target sell change one way only?

For example i want to use one cell only to add stock. What ever stock
arrives i just enter this number into the one cell and it changes. I have
the formula configured to change the stock held when stock in and stock out
change. But,


A B C D
1 PRODUCT NAME STOCK IN STOCK OUT STOCK HELD
2 Bananas 20 0 20
3 Apples 100 50 50
4 Pears
5
6 ADD STOCK ADD to STOCK IN
7 Bananas (enter number here)


I want to be able to enter the number of stock that i receive into one cell
so that all other cells change appropriately. For example if i get 20 more
bananas i want to enter 20 into cell "B7" so that this number will be added
to cell "B2". Thereafter if i change this number again when more stock comes
it adds again. Is this possible?