I don't believe there is an option to add drop down menus to Pocket Excel because that'd make it necessary to somehow rewrite Pocket Excel or at least to write a plug-in.
But let me ask what exactly do you wish to use this for? If you can somehow convince your costumers to install software on their phones, you might want to have a look at SoftMaker Office (which is easily the best Office packet for phones).
Otherwise, if your costumers often have to enter the same strings into cells in Excel, you might want to have a look at MagiKB Lite. This allows you to add keystrokes to the keyboard. For example, I myself used it to change the keyboard layout on the Redfly from English to German. But the important thing is that you can set it so it will input several words if a key is pressed for two seconds for example. So you could make it so that the users only have to press one key to enter text into Excel cells.