No. FileMaker Does Have a custom dialog box. When you double click on show custom dialog box there is a tab at top for Input Fields.
There is three input fields per dialog box
I'm sure someone has custom dialog box that has more features. You might google FileMaker custom dialog box.
Show Custom Dialog can specify up to three fields for user input.
For greater control over what appears in an input "dialog", you can use New Window to open a small modal window for user input. Such a window refers to a layout so you can use any standard FileMaker layout object including drop down lists, portals, buttons etc to construct the interface you want.
And I amsure that there are plugins available that provide better custom dialog options.