Most modern Android based devices support USB OTG function [1]. USB is a Host centric bus that means there is only one Host device and many different Devices can be connected to it. Using inexpensive USB-OTG cable it is possible to swich smarphone USB port from Device to Host, and connect external devices, if chipset and OS build allows this. Google provides detailed information on how to code USB Host [2] in your application.

[1] http://www.usb.org/developers/onthego/
[2] https://developer.android.com/guide/topics/connectivity/usb/host.html