How to export a wsp file from sharepoint 2007 and import it to sharepoint 2010. Also what all are things that we should keep in mind before & after the process
For extracting you can use Solution Extractor Tool from below link
Once you have extracted the wsp. You can use PowerShell for installing it into your SharePoint 2010 environment.
Add-SPSolution c:\path\wspname.wsp Install-SPSolution –Identity wspname.wsp –WebApplication http://sp2010url -GACDeployment
Ideal way is to upgrade your visual studio solution to 2010.
Theres a similar question & answer here regarding exporting wsp from SP 2007.
You could use Powershell to install in SP 2010. Refer this post
Eg: To add to farm
Add-SPSolution -LiteralPath "C:\FolderName\Solution.wsp"
Deploy to Site Collection
Install-SPSolution -Identity "Solution.wsp" -WebApplication "http://WebSiteUrl" -CASPolicies -GACDeployment -Local -Force
The parameteres are explained in the post. Note: CasPolicies is depricated in SP2013 in case you need to deploy in SP2013 later on.