First:
The documentation says "binary" though it looks more like hexadecimal because it says the ACK packet has to start with "0xFE 0x02".
Well, it's impossible print actual binary data in a human-readable form, so documentation will usually either give a sequence of hexadecimal bytes. They could use a bytes literal like b'\xFE\x02' or something instead, but that's still effectively hexadecimal to the human reader, right?
So, if they say "binary", they probably mean "binary", and the hex is just how they're showing you what binary bytes you need.
So, you need to convert the ASCII representation of a number into an actual number, which you do with the int function. Then you need to pack that into 8 bytes, which you do with the struct module.
You didn't mention whether you needed big-endian or little-endian. Since this sounds like a network protocol, and it sounds like it wasn't designed by Microsoft, I would guess big-endian, but you should actually know, not guess.
So:
imei_string = '1234567890123456789'
imei_number = int(imei_string) # 1234567890123456789
imei_bytes = struct.pack('>Q', imei_number) # b'\x11\x22\x10\xf4\x7d\xe9\x81\x15'
buf = b'\xFE\x02' + imei_bytes + seq_bytes
(You didn't say where you're supposed to get the sequence number from, but wherever it comes from, you'll pack it the same way, just using >H instead of >Q.)
If you actually did need a hex string rather than binary, you'd need to know exactly what format. The binascii.hexlify function gives you "bare hex", two characters per byte, no 0x or other header or footer. If you want something different, well, it depends on what exactly you want; no format is really that hard. But, again, I'm pretty sure you don't need a hex string here.
One last thing: Since you didn't specify your Python version, I wrote this in a way that's compatible with both 2.6+ and 3.0+; if you need to use 2.5 or earlier, just drop the b prefix on the literal in the buf = line.
