Would like ask about board, how it powers up for normal operation? If i connect board to a usb host, can it power up from usb 5v? If i connect another uext module (3.3v), how much current i can draw without damaging board's power supply?
The board consumes ~100mA @ 5V with the default demo program and nothing else connected to it. The typical USB port provides ~450mA (by default it should provide more by specification but that value gets lowered by the inner resistance of the USB cable). This means that you would have ~300mA for additional UEXT modules.
The board can be powered from several places. It can be powered from the mini USB connector and it can be powered from the Li-Po battery charger. It is completely fine to have both connected simultaneously but mind that under-charged batteries would increase the current consumption from the USB (since the board would attempt to charge the battery).
The board can also be powered from the ICSP connector.
Finally, if you require more current than what the mini USB or the ICSP connector can provide (and the battery is not an option or if it is excessive load on the power circuit) you can provide 5V external power supply to pin #1 of the EXT2 connector (pin #2 of the same connector is GND). This way the limiting factor would be the maximum current that SY8009AAAC is capable to provide. This is 1.5A. 1.5A - 0.1A = 1.4A maximum for your UEXT modules.