You can change your welcome section to a modal by going to Appearance > Styles > XenForo - UI.X > Style Property Groups > [UI.X] Welcome Section and checking the Welcome Section as Modal box.
Underneath this box, you will also see additional options for this modal. You can edit the position by changing the Welcome Modal Position value. Enter 0 for left and 1 for right. When a user chooses to close the welcome modal, you have the option to insert the content into the welcome section. To enable this feature, check the Insert Welcome Section Upon Closing Modal option. Your Welcome Modal's opacity can be edited with values from 0 to 1, 0 being no opacity and 1 being 100% opacity.