Python 2, 84 bytes

i=input()[::-1];o=""
for x in"kMGT ":o+=(i,i[0:3]+x)[len(i)>3];i=i[3:]
print o[::-1]

Try it online!

• Reverse the input
• Step through in threes appending the next letter
• Reverse and print

Not sure that this is the most efficient approach but it's the best I can come up with at the moment.