Although you do not give any useful information at all in your question about the file format or how many of these numbers there are and how they might be arranged, here are two possible solutions for you to try out.
Method One: from decimal value
Simply convert it into hours, minutes and seconds, and then sum them:
>> val = 175012.76;
>> hrs = floor(val/10000)
hrs = 17
>> mns = floor(rem(val,10000)/100)
mns = 50
>> scs = rem(val,100)
scs = 12.7600000000093
>> 60*60*hrs+60*mns+scs
ans = 64212.76
Method Two: read the file as characters
It might be easier to read the file using textscan, which would allow you to import the values as strings, which can then be parsed directly into hours, minutes and seconds. This is much faster and more efficient than separate calls to str2num, cellfun or the like: >> str = '175012.76'
str = 175012.76
>> vec = sscanf(str,'%2d%2d%f')
vec =
17
50
12.76
>> sum([60*60;60;1].*vec)
ans = 64212.76