If you want to add links to external links, there is a limitation.
Overexcessive links to other sites, not hubpages links, will be easily defined as overly promotional and HP will require you to revise your hubs.
In particular, the following actions are likely to get your Hubs identified as overly promotional:
• linking repeatedly to the same site within a Hub or across many Hubs
• including links to a page that contains largely the same content as your Hub
• linking to products or services unrelated to a Hub's content
• including more than 2 links to any one domain
If you want to add links of hubpages or your other hubs, the button ‘suggest links’ on the top of your hub help you add links to your other Hubs, other authors' Hubs, and topic pages. The link suggestion tool can be used to improve your HubKarma.