You cannot set 0.2.1-alpha automatically, but 0.2.1-alpha.0 is possible.
npm version supports a --preid option to specify the prefix of prereleases. It’s available in combination with pre* versions.
Example 1. Make the alpha of next major version:
# 1.2.3 => 2.0.0-alpha.0
npm version premajor --preid alpha
Example 2. Bump alpha to beta:
# 2.0.0-alpha.0 => 2.0.0-beta.0
npm version prerelease --preid beta
Once you create a prerelease, you can increment the number using prerelease argument.
# 2.0.0-beta.0 => 2.0.0-beta.1
npm version prerelease