OK, I'll give this a whack. The addition to OG would be straightforward if you knew the SG and volume of the juice you would get if you could separate it from the crud in the puree. The crud is ignorable - it won't contribute to fermentables, and will get separated out at some point in the process. The juice will contribute both sugars and water.
You don't say what kind of fruit is in the puree, but my experience is that most are in the 1.040 - 1.050 range (e.g., raspberry, apple). Obviously, grape is higher, but presumably that's not what you're using. Anyway, this gravity range is probably close enough to your base beer's OG that you could just ignore it. However, you should keep in mind that unlike the sugars from the beer portion, those from the fruit portion will be essentially 100% fermentable.