cat your_text_file | sed 's/<lex pos=\([^>]*\)>\([^<]*\)<\/lex>/\2_\1 /g' > output_file
But then, using XML tools is probably a bit more user-friendly... and safer, as it doesn't rely on the exact formatting. But I feel more comfortable with sed than with XSLT :)
PS What this expression does is to replace the whole line ('s' for substitute) by the matched sub-expressions (the bits between \(...\) - in reverse order, hence \2 ('time' in the example) and \1 ('NN'). The final 'g' means global, ie more than once a line if applicable. 'sed' can be a little daunting, but it is very powerful.