You can add Gaussian noise like this
noisyVec = vec + amplitude * randn(size(vec));
where vec is your other random signal.
I have no idea what you're talking about regarding a threshold. You say you need to "change my threshold value for the signal" -- well, what is the existing threshold value and how did you get it originally? Is it in a variable? WHY do you need to change it? Changing it is not necessary as far as I can see to add random numbers to a vector.