You can input them into Outlook, then sync the BlackBerry to the PC. Are you asking for a program that emulates the BlackBerry user interface and lets you type on the PC keyboard directly into the BlackBerry, as if you were typing on the BlackBerry keyboard? I'm unaware of any program that does that.
My old Palm would let you do what you're asking but not BB. Most folks seem to be using Outlook with BB. I used DM to convert Palm contacts with both devices connected to PC.
Posted from my CrackBerry at wapforums.crackberry.com
If you don't have Outlook (or just don't want to use it), you could also use the ASCII Importer/Exporter in conjunction with something like Excel or another spreadsheet that can save as an ASCII file. This would only be useful if you intend to enter the contacts in bulk. For one or two contacts at a time, I wouldn't recommend this approach.