append + at of each line only if...

I am trying to append end of each line with positive sign +, only if the last line of the character is 0, eg in the file1 below the first record end of the line is negative sign(-), i do not want to change anything for this line.
I have a file like below

FILE1.TXT
21111111110065200+B86860003OLFXXX007000020091224+0006520+0000652-
22222222220000653-B94030001OLFXXX020000020091210+0000000+0000000
23333333330000654-E71060001OLFXXX005000020091224+0000000+0000000
24444444440000655+E71060002OLFXXX015000020091210+0000000+0000000
21112223330000101+E71060002OLFXXX000000320091224+0000102+0000000
22222222220000201+E71060002OLFXXX000000320091224+0000000+0000000

I want the outfile FILE2.TXT like below in KSH script

21111111110065200+B86860003OLFXXX007000020091224+0006520+0000652-
22222222220000653-B94030001OLFXXX020000020091210+0000000+0000000+
23333333330000654-E71060001OLFXXX005000020091224+0000000+0000000+
24444444440000655+E71060002OLFXXX015000020091210+0000000+0000000+
21112223330000101+E71060002OLFXXX000000320091224+0000102+0000000+
22222222220000201+E71060002OLFXXX000000320091224+0000000+0000000+

what have you tried?

sed 's/0$/0+/' FILE1.TXT  > FILE2.TXT