Adding treacle will not give you more body. Most of those sugars will fill out, and you'll actually have a bit drier mouthfeel than you want.
You might want to consider priming with this.
http://www.muntons.com/homebeer/other-products/misc_kreamyx.asp
But honestly, like Bensiff said, if your beer's not carbed and conditioned then you really don't know what the mouthfeel and body will be like. Co2 goes a long way to lifting the percived body and the mouthfeel.
If this is a kit recipe or the first time you brewed it, I would prime, carb and condition as normal, before assuming something is wrong. You may be surprised. If not, drink it, brew it again and do something on the next batch to bump up the body/mouthfeel. Like carapils/Dextrin, or the afore mentioned creamy x.
I notice a lot of new brewers think something is wrong long before the beer is actually finished. It's a long journey the beer has to go through...and it changes so much on the journey, once it's been in the bottle for 6 weeks you'd be surprised how different the beer will be from what you perceived it to be now.