Hi there,
Unfortunately there is no easy way (depending on your perspective) to create high quality sculpts. In addition converting existing meshes to sculpties requires more processes to achieve.
That being said, yes it can be done, however you may need to invest in some software if you do not already have these programs.
If you have Zbrush or plan to get it, you can use a program called sculptie maker from
Vlad Bjornson. He has put a number of tools and tutorials on his page here.
http://www.shiny-life.com/sculpty-maker/ This method does not actually convert existing meshes to sculpts but rather allows you to use
a compatible mesh (for sculpts) to use your existing mesh to "project" onto the sculptie compatible mesh.
Also there is what looks to be quite an amazing plug in for 3ds Max called "Prim Composer" by
Shack Dougall.
http://liferain.com/downloads/primcomposer/ This one allows you to create SL Prims inside of 3ds Max as well as sculpties. I believe that you would use a similar projection method to create a sculpt of your existing mesh in this one as
well. The big news on this process is that you can create whole scenes complete with textures,
baked textures, shadows and lighting and then import them into SL where your scene or build
is then automatically constructed. There is alot more to it than my brief description here.
But you can get full details from the link listed above.
Neither of these processes can be described as simple as each of them requires a number of steps. While the tools and plugins are free the primary programs you will need to use them are
not.
So the barriers lie in the cost of the software as well as the effort required to use the methods.
But if you can master any of these techniques the upside is that it is definitely a marketable skill. Especially if your creating sculpted clothes as there are plenty of people in SL that would
buy high quality results. Initially it may be easier to use your existing meshes as references to
just create new sculpts of the clothes you want.