You can use this conversion table: http://roselab.jhu.edu/~raj/MISC/hexdectxt.html
eg, if you want a transparency of 60%, you use 3C (hex equivalent).
This is usefull for IE background gradient transparency:
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#3C545454, endColorstr=#3C545454);
-ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#3C545454, endColorstr=#3C545454)";
where startColorstr and endColorstr: 2 first characters are a hex value for transparency, and the six remaining are the hex color.