Reema, normally the diploma you have earned, is sufficient to begin work as a merchandiser, because you would already have learned about, fabric, construction, costing and production. What you may not have learnined in depth is the nuanaces of export marketing as well as skills in documentation, which is a crucial part in export/ import business.
Ideally if you have the, time and energy, you must go for the international business degree offered by Insititutes like the IIFT. But that would mean first going in for an undergraduate degree. Another option is to pursue a diploma in export/import managment offered by many universitiies in distance learning mode.
Work for a few years in smaller export firms, get some experince even while registering for a undergradate degree in correspondence. If you are working, go for some easier subjects like english literature. But a commerce degree might be more useful.