Hi there, I am a newbie on Motif XF and need a little assistance to understand how can we sample the sounds without using input source (A/D input or Microphone or CD).

The Motif XF can "sample" (the act of recording audio for playback triggered with a MIDI Note-On) either through:
1) the A/D INPUT (mic/line)
or
2) FW-audio input (Audio signal from the IEEE1394 connector (available when the optional FW16E has been installed) is recognized as the recording source. The audio signal here is transmitted from the computer via FW Ports 3 and 4 - aka "FW L" and "FW R"
or
3) RESAMPLE (where all audio sources are available including the PARTs of Motif XF assigned to L/R , AD Input, and the FW Input.

The act of LOADING data from a USB drive is not considered "Sampling" - but rather importing audio data.

I have samples available on USB flash drive (in WAVE Format) and how can I use sampler to import them?

As long as the samples are stereo or mono, maximum 44.1kHz, 16-Bit (.wav or aiff format). Audio at that starts as a .wav or aiff is imported to a Motif XF Waveform. As many as 256 samples can exist in any single Waveform... there are 128 Notes (C-2 through G8) and each can house a stereo sample (128 x 2 = 256).

The process of importing a .WAV is a function of [FILE] > Set the TYPE = .Wav or AIFF as necessary.
Select a Target for the data... you can import it to a Normal Voice or into a Drum Kit Voice. You do not specify what type of data you are importing and there are simply too many things to cover in a simply answer here.

Typically you import the data to a single KEY BANK = for example, "C3" once a sample has been imported you can then set a KEY RANGE and VELOCITY RANGE for that individual bit of audio. Pressing [INTEGRATED SAMPLING] will take you to the internal RAM of the XF where you can see your WAVEFORMS.

I am coming from Korg keyboards and much familiar their sampling structure but on Yamaha it seems slightly different. Also does anyone know if Motif can accept AKAI or SF2 format?

No, the Motif XF does not import AKAI or SF2 format samples directly.

Thanks, this has been really helpful. I will give it a try to import Wave file the set the key range. I will let you know if I run into any issues.

Regards