Regrettably, no. Maps purchased from the Avenza Map Store are locked to your device's internal storage in order to protect vendor content. This means that you can't store or move these maps to your SD card.
One thing to remember is that if you are logged in to your free Avenza Maps account any maps you purchase/download are associated with that account. This means that if you need to free up space on your device you can delete any of your maps and re-download them free of charge at a later date. If you need to use your maps on other devices, you can link up to five devices to your account and download your maps on to any of them.
Additionally, if you are loading maps from another source such as Dropbox, an internet URL or an email attachment you can set a storage location for these maps in the Avenza Maps settings. This will not apply to maps downloaded from the Map Store.