Replacing section of brake line

I’m performing some repairs on a Kia Rio MkII, after experiencing some brake issues I’m in the middle of rebuilding the rear right brake caliper and replacing the brake hose.

I was hoping to be able to just undo the brake union from the hose, replace, job done – how naive I was! The union is rusted good and proper, so at the very least the union needs to be replaced. There doesn’t appear to be enough brake line to cut the union off, add a new union and re-flare. A quick look under the car shows the next break in the line is at the other end of the vehicle at the front left, so replacing the whole length of line isn’t ideal.

I’m wondering how sound cutting a section of the end of the line off, adding a female union to the original and a male to some new line and running that to the hose would be? Is adding a join mid-line frowned upon, or in this instance is that a reasonable idea?

Thanks!