It is possible. It is just confusing and tiring. It is listed in the documentation these days .
Steps
Go to macports trac and find the package you need. This is the link for subversion , which is the package that I will use in the example.
Click PortFile 
Click "Edit Journal" (top right) 
Make changes until you find the version you need. Remember the revision number. 
In this example, I am looking for version 1.7 of subversion.
With the specified revision number. You need to check the version of this subdirectory in this revision.
cd /tmp svn co http://svn.macports.org/repository/macports/trunk/dports/devel/subversion --revision 106629
Then cd to the folder and run the installation
cd subversion sudo port install
Then it should be selected by default. You can check with
sudo port installed subversion The following ports are currently installed: subversion @1.7.10_0 (active) subversion @1.8.8_0 subversion @1.8.10_0
jmathew
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