You need to take them to a sewing shop that does alterations - they'll be able to tell if they can pinch fabric from e.g. inside the hems, pocket flaps, etc - but be warned, if the spare fabric has been inside your trousers, it may be a slightly different shade as the outsides may have faded. If the trousers are a standard colour/fabric, then you might be able to buy a remnant to add to them - but the sewing shop/alteration person will advise you. My friend does alterations and often has to rob bits of the insides of garments to extend them, but it's only by a little bit (unless your trousers are flares, LOL)