No, Foreign nationals can't secretly contribute financially to electoral campaigns, under the guise of being in corporations, because of Citizens United.
The law impacted by Citizens United was USC Title 2, Section 441b, which prohibited for profit corporations, labor unions, and other associations from contributing. This section specifically was found to violate the rights of free speech, as protected by the First Amendment, for a collection of individuals gathered via contractual agreement.
Beyond 441b, USC Title 2, Section 441, Subsection E, prohibits foreign nationals from contributing to political campaigns. This section wasn't addressed by the Citizens United ruling. “A foreign national” is defined to include any “partnership, association, corporation, organization, or other combination of persons organized under the laws of, or having its principal place of business in, a foreign country.” Even further, the scope of those considered "Foreign nationals" includes non-resident aliens. Even if they located their business in the US, unless they became US citizens they would be subject to the restrictions of FEC regulation 11 CFR 110.20(i), stating:
A foreign national shall not direct, dictate, control, or directly or indirectly participate in the decision making process of any person, such as a corporation, labor organization, political committee, or poltiical organization with regard to such person’s Federal or non-Federal election-related activities, such as decisions concerning the making of contributions, donations, expenditures, or disbursements in connection with elections for any Federal, State, or local office or decisions concerning the administration of a political committee.