Question about Hauck Open'n Stop Safety Gate

Yes, that's exactly what I did, with two 21cm extensions to reach 120cm. You can also mix extensions of different lengths (9cm or 21cm) to adjust to the desired length. However, the more extensions you add, the less rigid the whole is.

I think you can add extensions indefinitely.

I don't know, but there is no extension cord in the package. You'd have to look on the manufacturer's website.