Feed microbes by adding dextrin?

I was hoping for a 1.025 FG, but i didn't mash hot enough and ended up with 1.015 FG. Then I racked to secondary and added the microbes (Roeselare) at 1.015 SG.

Should I add enough dextrin to raise the SG from 1.015 to 1.025? I want the microbes to have something to feed on, and I don't want my finished product to be too dry. I'm at the beginning of the 12 month souring process. I will be adding sour cherries in a year from now.

Dextrin will help but I would not be very worried about the gravity at this point. I would taste the beer in about 6 months and you can feed the bugs then.
I am a fan of periodic feeding. This way you can make sure the slow growing bugs get some food. Feeding at the 6 to 9 month mark will help feed the pedio which will greatly increase the sourness. It is also when the Brett is working too.

i forgot to mention that this is a brown ale. 1.065 OG.

Add Sour cherry? This flavor naturally develops about 2 years into the fermentation in a flanders. You don't add cherry's like you do for a Kreik Lambic.

you can add fruit to sour beer. fo sho.

I think what was meant is that Oud Bruin's (looks like the style of beer you are making) don't typically have cherries added for the cherry flavor, it comes from the Brett during aging.
But there is definitely no problem adding fruit to this beer. When you add it will really make a difference. For Kriek's the cherries are added a few months before it will be bottled and usually after the beer has been aged for a while. That helps to ensure that the cherry aroma/taste is very fresh.
And adding it earlier on will most likely give you a mild cherry flavor layer and some tartness. And should help build some complexity.
The Mad Fermentalist put some of his Flander's Red on cherries and thought it improved the beer considerably.

indeed. my recipe is fashioned after his.

i will feed the dextrin in increments based on when the different microbes are supposed to kick in. that sounds like a good idea.

So is there a general rule of thumb on how much and when to add maltodextrin?

I'm not sure if there are "rules of thumb" but I can tell you what I have come up with.
If you look at the chart of Raj Apte's site (near the bottom), it shows in a mixed culture the time range of the dominant microbe.
http://www2.parc.com/emdl/members/apte/flemishredale.shtml
For me, I taste at 6 months, 9 month and a year. At the 6 month date, if the sourness is pretty low I will add .5 - 1 lb maltodextrin depending on my feeling. Then at the 9 month point, I will see how the beer is progressing and let it go or add fruit. The amount depends on the type of fruit and the flavor or extra acidity I want. I don't add any flavors until I have tasted the beer at the 6 or 9 month mark because until then I don't know what direction the bugs have taken the beer (I have a guess while designing the recipe). I try to match my fruit or other flavoring component to compliment a flavor I'm tasting in that beer. And sometimes the beer doesn't need anything.
This is just my method I have developed but I am still in the early learning stages.