If you want to plug USB devices (USB sticks, keyboard,…) to your Samsung Galaxy i9100 S2 you can do this. Even for more sophisticated devices like inertial measurement units (IMU). But you will have to enable the USB host mode for this.
For the physical connection, a USB micro-A (5-pin) to USB A female adapter is necessary. Note that it really has to be a 5-pin adapter. Usual 4-pin adapters would not be able to recognise the USB client device. For example this one works.
If your USB device is not recognised (or only as an unknown device) this may be due to the missing drivers. Your next step is then the installation of the respective drivers – e.g. a serial to USB converter for an xsens IMU which is based on a FTDI chip set. If there is no compiled driver available for the required kernel version on the SGS2, it is necessary to compile the drivers for the kernel and to include them.
Another possible cause for your device not being recognised is that your device is not allowed to be used. According to the USB OTG specification, USB devices supported by the USB OTG host have to be specified in a white-list, the so called targeted peripherals list. If your device isn’t, again you will have to modify the kernel. Either you add it to this white-list, or you disable this white-list altogether.
The process for editing the kernel is described in the following post.