I don't know if you can convert right away, syllabus for MPL is completely different from the traditional CPL(IR/ME) training.
Firstly, regardless of whether you are in the MPL or traditional training set up you can't get your ATPL until you have 1500hrs (well in Canada that's the number, not sure about elsewhere, but I think 1500hrs is pretty standard). People come out of flight school with FROZEN ATPLs meaning that they've done the theory and written the exams, but they don't actually have the license and won't get it till they gain the necessary practical experience.
Secondly, once you graduate with an MPL that's all you have. You will only have that license and will have to find an airline that will be hiring MPL pilots. However, having said that the number of airlines looking for MPL trained pilots is increasing day by day and I frankly would not be that worried about leaving flight school with an MPL. Frankly its difficult as it is with a traditional CPL.
Can you convert straight away to a CPL? No I don't think so, like I said the training is very different. The whole point of the CPL is that you will be able to get into a Cessna and fly single-pilot IFR which MPL training simply doesn't prepare you for. I was at a college during a time where there were MPL pilots doing their course at the same time as me and they were on a completely different syllabus from us, with very little resemblance to a traditional ppl/cpl course.
I suggest you take this question over to the Flight Training section, there will be people there with a lot more knowledge about these kinds of things.