You will need to use some conditional statements, if you are setting an attribute value in a change activity in a microflow, it would look like this:
if OldAttributeValue = false then
If you want to have old value combined with new value, you'll need to use nested if then statements.
If you are building a string variable in your microflow, you could accomplish the same thing with Exclusive Splits
Hope that helps.
You could use an 'if' statement, saying that if the boolean is true, then the string should 'true'.