Can I negotiate with the card issuer to extend the low interest period on a balance transfer card?

You can negotiate. Simply request to your credit card company politely. You can call or fix an appointment. Credit card companies often value a potential customer. If you past payment records are good, then you may get a positive reply.

I agree with Patricia..
But you need to keep in mind few things...pay your balance always in full and on time. In case of any late payment, don't hesitate to call your customer service representative. Try not to exceed 35% of your credit limit. If you’re maxing out your credit limit, it can negatively impact your credit score.

I don't think it will work. The teaser rate stretches between 12 months and 18 months. The interest rate kicks in after that period. It's their time to make money. Why would they agree to extend the period? You have to negotiate really hard

To enjoy the benefits of a balance transfer card with low interest rate, it is better to plan financially how to repay the outstanding balance within the teaser period. Try to do it to enjoy the real benefit.