How to post-process aggregate values

Dear All,
I need help in implementing post processing for aggregate values.

I have a column (lets call it NTIME) which stores number of seconds. When I display the column data I have a post-processing step (using the 'value' attribute of the text element) to convert it into a display string like "xx min yy sec". In the value attribute I use a formula like (=INT([NTIME]/60)&" min "&INT(MOD([NTIME];60)&" sec"&) ), which works with no hassle.

Now I want to show the sum of the column and want do the same post-processing for displaying the sum like this:
row 1: 1 min 12 sec
row 2: 1 min 24 sec
Group Footer: 2 min 36 sec

I have the numeric sum calculated using the aggregate function. But to apply the display format function I need to refer to the sum in the display formula which I am unable to find. So currently my report only displays the numeric value like this:
row 1: 1 min 12 sec
row 2: 1 min 24 sec
Group Footer: 156

If anybody has come across similar scenario before and if any solutions are known please let me know how can I achieve this.