I too am curious as to why this has not been dealt with within the existing IVA. Is the liability you refer to significant, and have you asked your current IP to see whether they are prepared to put forward a variation for you of the original terms to seek the inclusion of this creditors - with their prior permission of course.
I have a feeling that you were posting about this earlier on the forum, and that there is a logic to your strategy, but do bear in mind that your original IP has to terminate the existing IVA firstly, and may just do that by petitioning for your bankruptcy first in the event of failure.
I would tread carefully if I were you!
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk