Hi,
that is a tricky matter, the classes you refer to is part of the .NET Framework. 
Any related function (such as your System.Security.Cryptography.SHA1) needs to be exposed by a COM visible object in order to be used from PB Classic.

Instead, the link provided by Roland points to Win32 functions, that are NOT .NET managed libraries. Win32 functions are legacy, old and not as safe/secure as .NET libs.
So, if you can find there a function equivalent to SHA1 that computes the hash for the given input data, you are fine.

But it is not the same as using the .NET System.Security.

If you want to go for a native .NET library you have to:
1) build up a COM Visible wrapper from Visual Studio, have some background knowledge here:

https://community.appeon.com/index.php/articles-blogs/tips-tricks-techniques-articles/17-powerbuilder/167-calling-net-components-from-powerbuilder-via-com-wrappers-redux

2) Once you have registered the COM object you can use the PB ConnectToNewObject to instantiate it and use the functions that you have wrapped (such as SHA1Managed).

Cheers,
Marco

Try this:

http://www.topwizprogramming.com/freecode_bcrypt.html