The addition itself is very simple, you just need to add the browsers in the correct format, that looks like this:
[IE 6.0]
browser=IE
Version=6.0
majorver=#6
minorver=#0
frames=True
tables=True
cookies=True
backgroundsounds=True
vbscript=True
javaapplets=True
javascript=True
ActiveXControls=True
Win16=False
beta=False
AK=False
SK=False
AOL=False
Update=False
[Mozilla/4.0 (compatible; MSIE 6.0; Windows 98*)]
parent=IE 6.0
platform=Win98
beta=True
[Mozilla/4.0 (compatible; MSIE 6.0; Windows NT*)]
parent=IE 6.0
platform=WinNT
beta=True
[Mozilla/4.0 (compatible; MSIE 6.0)]
parent=IE 6.0
[IE 6.0b]
browser=IE
Version=6.0
majorver=#6
minorver=#0
frames=True
tables=True
cookies=True
backgroundsounds=True
vbscript=True
javaapplets=True
javascript=True
ActiveXControls=True
Win16=False
beta=True
AK=False
SK=False
AOL=False
Update=False
[Mozilla/4.0 (compatible; MSIE 6.0b; Windows 98*)]
parent=IE 6.0
platform=Win98
beta=True
[Mozilla/4.0 (compatible; MSIE 6.0b; Windows NT*)]
parent=IE 6.0
platform=WinNT
beta=True
[Mozilla/4.0 (compatible; MSIE 6.0b)]
parent=IE 6.0b
The fun part will probably be finding your browscap.ini. It's location depends on your server.
In a windows server look for it in %systemroot%\WINNT\System32\Inetsrv folder.
In a Linux Server, you might want to put it in /etc/extra/
Then go to your php.ini file and declare it like this: browscap = /etc/extra/browscap.ini
And finally reboot your server.
If you are using a rented server you might not have access to the file and will need to contact your hosting provider to do any changes.
Finally, once you locate the file (or created it if it doesn't exist) just edit it with notepad (or text edit if you're a mac user)
Good luck! I hope this helps.