|I'm not 100% sure what you are trying to do, but I'll toss these 2 suggestions...|
1 - If you click and hold on a cell that contains a hyperlink you can edit it.
2 - You can use the concatenation operator & to combine strings.
=A1 & "_target="blank""
You'll have to play with the quotes. I don't have Excel available to me right now so I can give you the exact formula.
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